Corn–Soybean Rotation Effects on Nitrate Leaching

Because agricultural production is a primary nonpoint source of NO− 3 in the nation's waters, a better understanding of the effects of cropping systems on NO− 3 leaching is required for developing agricultural production practices that reduce NO− 3 leaching. A 4-yr experiment was conducted to study the effect of a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation on NO− 3 leaching using passive capillary fiberglass wick lysimeters. The experiment had five N rates on corn (0–200 kg N ha−1 in 50-kg increments). Corn was planted in 1997 and 1999, and soybean was planted in 1998 and 2000. The increase of soil residual NO− 3–N concentrations in the surface 25 cm of soil after crop harvest was not significant (p = 0.05) when N fertilizer rates applied to corn increased from 0 to 100 kg ha−1 but was significant when N rate increased from 100 to 200 kg ha−1 in the corn years. The 2-yr average soil residual NO− 3–N concentrations and annual flow-weighted NO− 3–N concentrations in leachate were significantly higher (p = 0.05) in soybean years than in corn years at 0 and 100 kg N ha−1 applied to corn, but the differences at the 200 kg N ha−1 rate were not significant. The results indicate that at recommended N rates applied to corn in a corn–soybean rotation, NO− 3 leaching potential is similar for corn and soybean, but at less than 100 kg N ha−1 rate, a greater NO− 3 leaching potential exists under soybean than under corn.